With the dramatic increase in communication demand in recent years, the use of large capacity and high-speed photonic networks has increased in access networks for transferring data between base stations of carriers and user sites. Wavelength Division Multiplex (WDM) systems have been studied for the replacement of conventional Time Division Multiplex (TDM) systems and are coming into practical use as next generation optical access network systems.
A Semiconductor Optical Amplifier (SOA) is as an attractive device to be used in such a WDM system. Comparing with an optical fiber amplifier module most commonly used nowadays, the configuration of the SOA is very simple and advantageous in dimensions and amplification wavelength bandwidth. Thus, the application of the SOA in various kinds of photonic networks such as a next generation optical access networks has been studied. Therefore, a semiconductor optical amplifier and an optical amplification apparatus with low polarization dependences are desired.
Related art is disclosed in Japanese Laid-open Patent Publication No. hei3-287237, and Japanese Laid-open Patent Publication No. 2002-151794.